In media having a constant index of refraction, the optical path length may be defined as the product of the geometric distance through which an optical wavefront traverses and the refractive index of the media. In contrast, as an image or optical signal propagates through a turbulent media random fluctuations in the index of refraction of the media results in variations of the optical path length. For example, an image propagating through the atmosphere encounters localized variations in the refractive index of the air. As such, the optical path length of a media having a varying index of refraction may be defined as the integral of n xcex4s, where xcex4s is an element of length along the path, and n is the local refractive index. Variations in the optical path length leads to randomization of the phase front contour of the wavefront, thereby causing the image to be obscured. This phenomena is known as phase-shifting.
FIG. 1 shows a representation of an optical signal propagating through a space. As illustrated, the exemplary wavefront or optical signal 1 may be segmented into four quadrants 1A, 1B, 1C, and 1D. As the wavefront 1 propagates through the space 2, quadrants 1A, 1B, 1C, and 1D are subjected to the optical characteristics of the space 2. For example, the index of refraction of the space 2 may not be uniform. As shown in FIG. 1, the space 2 may include areas of uniform refractive properties 3A, 3B, and 3C, as well as one or more areas of variable refractive properties 3D. As such, the optical path length the wavefront 1 traverses is not uniform, thereby distorting, defocusing, or otherwise compromising the wavefront 1 as it propagates through the space 2. As shown in FIG. 1, when quadrant 1D of the wavefront 1 is subjected to a higher refractive index 3D of the space 2, quadrant 1D traverses a greater optical path length than the neighboring wavefront quadrants 1A, 1B, and 1C. As such, the output wavefront 4 undergoes phase-shifting. As illustrated, the output wavefront 4 includes quadrants 4A, 4B, and 4C which are substantially in phase having traversed an equal optical path length. However, at a time t, quadrant 4D has traversed an optical path length less than the adjoining quadrants 4A, 4B, 4C, and is, thus, out of phase with the adjoining quadrants 4A, 4B, 4C.
In response, a number of systems and techniques have been developed to restore the original phase state of optical signals. One system which has been developed employs adaptive optics or active optical control systems to address variations in optical path lengths. Typically, active optical systems make use of adaptive optical elements that are based on mechanical implementation. One example of an adaptive optical element is a deformable mirror. The deformable mirror includes a distortable substrate having a light-reflecting material applied thereto. The substrate includes a number of small actuators coupled thereto which push or pull segments of the substrate, thereby reconfiguring the shape of the deformable mirror. In doing so, the actuators compensate for the distortions in the beam phase by making some parts of the optical path shorter and some parts of the optical path longer.
While present adaptive optics or active optical control systems have proven useful in many applications, a number of shortcomings have been identified. For example, these systems transform what fundamentally is an optical problem into a mechanical problem. As such, the mechanical systems used to reconfigure the deformable mirror may introduce jitter or noise into the signal. In addition, the response times of these systems may be unacceptably slow for high date rate applications. For example, response times in the range of megahertz to several gigahertz are not uncommon.
Thus, in light of the foregoing, there is an ongoing need a system capable of rapidly correcting for phase-shifting errors in optical signals.
The various embodiments of the optical system disclosed herein enable a user to easily correct for phase-shifts and aberrations in optical signals. Furthermore, the various systems disclosed herein permit optical corrections of incoming signals, thereby reducing jitter and aberrations associated with presently available mechanical systems.
In one embodiment, the present application is directed to a high speed optical system and discloses a photodiode which is sensitive to a wavelength of light, an image source irradiating a wavefront of a first wavelength on the photodiode to which the photodiode is sensitive, the wavefront containing an optical path difference induced phase-shift, a read source irradiating photons of a second wavelength on the photodiode to which the photodiode is insensitive, an electric field across the photodiode in excess of a breakdown voltage thereof and configured to result in an avalanching of electrons in the photodiode when the photons from the first source strike the photodiode, the avalanching electrons resulting in a photorefractive response which changes the index of refraction in the photodiode, and a capture device in optical communication with and configured to capture light reflected from the photodiode.
In an another embodiment, the present application is directed to a high speed optical system and discloses an InGaAsP photodiode which is sensitive to a wavelength of light, a first source of photons configured to transmit a wavefront at a first wavelength to which the photodiode is sensitive incident on the photodiode, the wavefront having an optical path difference induced phase-shift, a second source of photons at a second wavelength to which the photodiode is insensitive incident on the photodiode, an electric field across the photodiode in excess of a breakdown voltage thereof and configured to result in an avalanching of electrons in the photodiode when the photons from the first source strike the photodiode, the avalanching electrons resulting in a photorefractive response which changes the index of refraction in the photodiode, and a capture device in optical communication with and configured to capture light reflected from the photodiode.
In still another embodiment, the present application is directed to a high speed optical system and discloses an InGaAsP photodiode having a bandgap, the photodiode configured to operate in Geiger mode, a first photon source configured to transmit a wavefront at a first wavelength to which the photodiode is sensitive incident on the photodiode, the wavefront having an optical path difference induced phase-shift, the first wavelength less than the bandgap of the photodiode, a second photon source configured to emit light of a second wavelength, the second wavelength greater than the bandgap of the photodiode, a beam combiner positioned within an optical path and configured to combine the first and second wavelengths, an electric field applied across the photodiode greater than a breakdown voltage thereof, the electric field configured to result in avalanching of electrons in the photodiode when photons from a first photodiode are incident thereon, the avalanche of electrons resulting in a photorefractive response within the photodiode, and a capture device in optical communication with and configured to capture modulated light reflected from the photodiode.
The present application further discloses various methods for optically correcting for phase-shifting. One method disclosed in the present application includes baising a photodiode to operate in Geiger mode, irradiating a photodiode with a first wavelength of light to which the photodiode is sensitive, the first wavelength of light transmitting a wavefront, irradiating the photodiode with a second wavelength of light to which the photodiode is insensitive, correcting for a phase-shift of the wavefront with the photodiode by modulating light reflected from a surface of the photodiode with a photorefractive reaction within the photodiode, and capturing the modulated reflected light containing a corrected wavefront.
In another embodiment, the present application discloses a method for correcting for phase-shifting and includes configuring a photodiode to operate in Geiger mode, irradiating a photodiode with a first wavelength of light transmitting a wavefront, initiating a photorefractive reaction within the photodiode with the first wavelength of light, irradiating the photodiode with a second wavelength of light to which the photodiode is insensitive, modulating light reflected from a surface of the photodiode with the photorefractive reaction within the photodiode to correct for phase-shifting in the wavefront, and capturing the modulated reflected light.
Other features and advantages of the embodiments of the high speed optical system disclosed herein will become apparent from a consideration of the following detailed description.